Phased array ultrasonic imaging systems have been used for producing real-time images of internal portions of the human body. Such systems include a multiple channel transmitter and a multiple channel receiver, either coupled to a single array of ultrasonic transducers using a transmit/receive switch, or coupled separately to a transmit transducer array and a receive transducer array. The ultrasonic transducers, placed in contact with the body, respond to short electrical pulses and emit corresponding pressure waves. The electrical pulses are applied to the individual transducers in a predetermined timing sequence so that the pressure waves, generated by the transducers, are phased to form a transmit beam that propagates in a predetermined direction from the array.
As the transmit beam passes through the body, portions of the acoustic energy are reflected back toward the transducer array from tissue structures having different acoustic characteristics. An array of receive transducers (which may be the same as the transmit array) converts the reflected pressure pulses into the corresponding electrical pulses. Due to different distances, the ultrasonic energy reflected from a tissue structure arrives at the individual transducers of the array at different times. Each transducer produces an electrical signal that is amplified and provided to a processing channel of a receive beamformer. The receive beamformer has a plurality of processing channels with compensating delay elements connected to a summer. The receive beamformer selects the delay value for each channel to collect echoes reflected from a selected focal point. Consequently, when the delayed signals are summed, a strong signal is produced from signals corresponding to this point, but signals arriving from other points, corresponding to different times, have random phase relationships and thus destructively interfere. Furthermore, the beamformer selects the relative delays that control the orientation of the receive beam with respect to the transducer array. Thus, the receive beamformer can steer the receive beam to have a desired orientation and focus it at a desired depth. Both steering and focusing can be performed dynamically.
To collect imaging data, the transmit beamformer controls the transducer array to emit ultrasound beams along multiple transmit scan lines distributed over a desired scan pattern. For each transmit beam, the receive beamformer connected to the transducer array synthesizes a receive beam by using the selected delay. The transmit and receive beams form a round-trip beam (i.e., a "center of mass" beam). The round-trip beams are synthesized over a predetermined angular spacing to create a wedge-shaped scan pattern, or a predetermined linear spacing to create a rectangular scan pattern.
Conventionally, to increase image resolution, the system has to increase the number of round-trip beams that are generated over the image sector. The required sampling (i.e., the number of beams) is in accordance with the Nyquist sampling theory, described in U.S. Pat. No. 5,431,167, which is incorporated by reference. When the sampling is increased, the overall time necessary to obtain the data and to generate the image increases. Furthermore, to create a three-dimensional image, the system has to collect acoustic data over three-dimensions, which increases the acquisition time. However, to image a moving organ, such as the heart, it is important to generate an image as fast as possible, and thus it may be necessary to increase the frame rate (i.e., the number of images generated per unit time) as compared to a conventional system. The increased frame rate avoids image blurring caused by the moving organ. The imaging system can increase the frame rate by faster operation, or by decreasing the number of lines (i.e., the number of beams) employed to produce the image; this in turn reduces the overall resolution of the image. Thus, in a conventional system, there is a trade-off between the resolution and the frame rate.
To increase the frame rate, a conventional imaging system can simultaneously synthesize two beams for each transmit beam over a selected sector. However, the generated images have frequently "artifacts", which are visual anomalies that appear in the displayed image, but are not present on the imaged object. These image anomalies occur due to, for example, a partially blocked aperture or when the round-trip beams don't have the same beam profile. The imaging systems can use a low pass filter that in fact "averages" signals of neighboring receive beams to remove the artifacts. The averaging, of course, reduces the resolution. It may be useful to design an imaging system that does not use this filtering to remove the artifacts.
An imaging system of another type may further increase the frame rate by simultaneously synthesizing more than two receive beams for each transmit beam and combining two receive beams synthesized from subsequently emitted transmit beams. Specifically, the transmit beamformer generates a transmit beam, and then the receive beamformer synthesizes four receive beams from the echos. An interpolator interpolates two synthesized receive beams to obtain a combined beam oriented along a selected direction. When synthesizing four receive beams, the imaging system obtains two interpolated receive beams for each transmit event; this technique is also called 4.fwdarw.2 parallel technique. The image data is formed from the two round-trip beams that again depend on the transmit beams and the corresponding receive beams. However, in this technique, the transmit and receive beams do not have a purely translational symmetry; there is also a mirroring component of the symmetry. The mirroring component may cause non-uniform profiles of the interpolated, round-trip beams when part of the transducer aperture is blocked (for example, by a rib) even though nominally the interpolated beam is aligned along the correct direction. Thus the partially blocked aperture may cause both an amplitude non-uniformity and a steering non-uniformity, both which can still be seen as a line-to-line artifact. While the 4.fwdarw.2 parallel technique works very well and provides a significant improvement over the prior art, there is still a need to eliminate some artifacts. Furthermore, there is also a need to further increase the sampling rate.
Therefore, there is a need for a phased array imaging system that operates at a high sampling rate, reduces the artifacts and is capable of producing two-or three-dimensional images of moving body organs.